CMP is a process of removing unwanted conductive or dielectric materials from a substrate by the application of chemical and mechanical forces. For example, during a damascene process, a metal such as Cu is deposited in openings within a dielectric layer. However, excess metal is deposited and must be removed with portions of dielectric material so that a top surface of the dielectric layer becomes coplanar with a top surface of the new metal layer.
A CMP tool is typically comprised of a polishing station with a rotatable platen on which a polishing pad is affixed. A substrate having a front side with one or more layers to be planarized has a backside that is held to a head piece by a vacuum during loading/unloading and by a retaining ring during polishing. After the substrate's front side is brought into contact with the polishing pad, a slurry is introduced and includes a chemical component such as an aqueous base or aqueous acid, and abrasive particles. As the polishing pad is rotated in one direction, the substrate typically rotates in the same direction, and the slurry effectively etches and abrades away a portion of the uppermost layer or layers on the substrate. During a polishing cycle, end point signals from an optical detection system may be analyzed either in real time or off-line to determine the end of the polishing cycle. Moreover, metrology measurements may be taken at the end of a cycle to indicate a final non-uniformity of film thickness across each wafer and may be used to adjust the polish process in subsequent polish cycles.
In some cases, CMP processes require a second polishing cycle where the wafer is reloaded into a CMP tool and non-uniformity thickness data from the initial polish cycle is used to determine backside pressures applied to the wafer in the second polish cycle with the aim of improving the final thickness non-uniformity. Sometimes a second polish is not possible due to a situation where final thickness constraints have already been met after the initial polish. In other words, further polishing of the thicker regions on the substrate cannot occur since the thinner regions that are already at the minimum thickness specification would also be thinned. Therefore, a CMP process that enables pressure adjustments during a single polish cycle is needed to minimize the difference between clearing times between different annual regions at the end of the polish cycle which reduces process time and improves film thickness uniformity across the substrate thereby increasing product yield.